The BRCA1-RAP80 complex regulates DNA repair mechanism utilization by restricting end resection.

The tumor suppressor protein BRCA1 is a constituent of several different protein complexes and is required for homology-directed repair (HDR) of DNA double strand breaks (DSBs). The most recently discovered BRCA1-RAP80 complex is recruited to ubiquitin structures on chromatin surrounding the break. Deficiency of any member of this complex confers hypersensitivity to DNA-damaging agents by undefined mechanisms. In striking contrast to other BRCA1-containing complexes that are known to promote HDR, we demonstrate that the BRCA1-RAP80 complex restricts end resection in S/G(2) phase of the cell cycle, thereby limiting HDR. RAP80 or BRCC36 deficiency resulted in elevated Mre11-CtIP-dependent 5' end resection with a concomitant increase in HDR mechanisms that rely on 3' single-stranded overhangs. We propose a model in which the BRCA1-RAP80 complex limits nuclease accessibility to DSBs, thus preventing excessive end resection and potentially deleterious homology-directed DSB repair mechanisms that can impair genome integrity.

The Rap80-BRCC36 de-ubiquitinating enzyme complex antagonizes RNF8-Ubc13-dependent ubiquitination events at DNA double strand breaks.

DNA double strand breaks (DSBs) initiate reversible cellular checkpoint and repair activities. Whereas many of the activating events at DSBs have recently been elucidated, the mechanisms used to terminate responses at these sites are largely undefined. Here we report a pathway required to reverse RNF8-Ubc13 dependent ubiquitination events on chromatin flanking DSBs. Inhibition of the Rap80-BRCC36 de-ubiquitinating enzyme complex partially restored DSB-associated ubiquitin levels following RNF8 knockdown or proteasome inhibition. Similarly, BRCC36 knockdown or expression of a BRCC36 de-ubiquitinating enzyme-inactive mutant rescued both 53BP1 recruitment to DSBs and ionizing radiation-induced gammaH2AX ubiquitination following RNF8 depletion, and mitigated ionizing radiation sensitivity resulting from RNF8 deficiency. Thus, concomitant and opposing RNF8-Ubc13 ubiquitin ligase and Rap80-BRCC36 ubiquitin hydrolysis activities are responsible for determining steady-state ubiquitin levels at DNA DSBs. These findings reveal a Rap80-BRCC36 dependent pathway that is required for appropriate DSB recruitment and repair responses.

The expression of the cytoskeletal focal adhesion protein paxillin in breast cancer correlates with HER2 overexpression and may help predict response to chemotherapy: a retrospective immunohistochemical study.

Paxillin, a cytoskeletal focal adhesion adaptor protein, has been shown to be transcriptionally up-regulated and phosphorylated by human epidermal growth factor receptor-2 (HER2) signaling in vitro. Paxillin expression may also correlate with HER2 amplification in breast cancer patients. In the current study, we sought to explore the relationship further between paxillin expression and clinicopathologic features and clinical outcome in breast cancer. A total of 314 primary invasive breast carcinomas were assessed for paxillin expression via immunohistochemistry. Paxillin immunoreactivity was compared with estrogen receptor/progesterone receptor status, HER2 status by silver in situ hybridization, age, tumor size, stage, Bloom-Richardson grade, nodal status, disease-free survival (DFS), and overall survival (OS). Paxillin expression was identified in 27.7% of breast carcinomas as diffuse cytoplasmic staining and the expression correlated with HER2 overexpression (p < 0.001). The influence of paxillin on clinical outcome, in particular the response to chemotherapy, appeared to differ depending on the HER2 status of the tumor. For the subset of HER2 nonamplified cases treated with chemotherapy, patients whose tumor showed a loss of paxillin expression demonstrated a significantly lengthened DFS and OS. In contrast, loss of paxillin expression in the HER2 amplified subset of patients who received chemotherapy correlated with a significantly worse outcome. These data suggest that paxillin up-regulation may be a part of the HER2 pathway in some breast cancers and, furthermore, paxillin expression may also influence the clinical response to chemotherapy, depending upon the HER2 status of a given patient's tumor. Further study of a role for paxillin expression in predicting response to cytotoxic regimens or targeted treatments is warranted.

Breast cancers with brain metastases are more likely to be estrogen receptor negative, express the basal cytokeratin CK5/6, and overexpress HER2 or EGFR.

Brain metastases (BM) from breast cancer are associated with significant morbidity and mortality. In the current study, we have examined a cohort of breast cancer patients who went on to develop BM for clinical-pathologic features and predictive markers that identify this high-risk subgroup of patients at the time of diagnosis. The primary tumors from 55 patients who developed BM were used to construct a tissue microarray. The clinical and pathologic features were recorded and the tissue microarray was stained for estrogen receptor, human epidermal growth factor receptor 2, cytokeratin 5/6, and epidermal growth factor receptor by immunohistochemistry. This cohort of patients was compared against a group of 254 patients who remain free of metastases (67 mo mean follow-up), and another cohort of 40 patients who developed mixed visceral and bone metastatic disease without brain recurrence over a similar period of time. Breast cancer patients who went on to develop BM were more likely to be <50 years old (P<0.001), and the primary tumors were more likely to be estrogen receptor negative (P<0.001) and high grade (P=0.002). The primary tumors were also more likely to express cytokeratin 5/6 (P<0.001) and epidermal growth factor receptor (P=0.001), and to overexpress human epidermal growth factor receptor 2 (P=0.001). The data presented above suggest a profile for breast cancer patients at increased risk for developing BM. Predictive factors to help identify patients with metastatic breast cancer who are at an increased risk for developing central nervous system recurrence might allow for screening of this population for early detection and treatment or for the development of targeted strategies for prevention.